1. Field of the Invention
The present invention relates to a sheet cutting apparatus adapted to cut edge portions of a half-folded sheet bunch which is folded in a shape of a brochure so that the edge portions are aligned, and to a sheet post-processing apparatus having the sheet cutting apparatus, and an image forming system.
2. Description of the Related Art
In image forming apparatuses, such as a copier, a printing apparatus, or a facsimile machine and image forming systems such as a bookbinding machine, various kinds of sheet processing have been realized in recent years. In post-processings, such as saddle stitching bookbinding or case binding, edge portions of a half-folded sheet bunch which is folded in the shape of a brochure using a folding processing mechanism (center-folding unit) are cut by a sheet cutting apparatus (trimmer unit) so that the edge portions are aligned. For example, as illustrated in FIG. 12, a half-folded sheet bunch 301 which is folded about a half-folded portion (left edge portion of FIG. 12) in the shape of a brochure has edge portions (fore edge which is a right edge portion of FIG. 12) 301a which form a v-shaped overhanging curved surface. When a user turns a page with his/her finger, the turning of pages is stopped at the center page so that it may be difficult to turn a page when the page is in the latter half of the brochure. Therefore, conventionally, a cutting member provided in the sheet cutting apparatus is used to cut the edge portions (fore edge) 301a of a spread of the half-folded sheet bunch 301 described above so that the edge portions are substantially aligned and form a flat surface to perform satisfactory bookbinding (Japanese Patent Application Laid-Open No. 2006-82153 and Japanese Patent Application Laid-Open No. H05-169396).
When such a sheet cutting apparatus (trimmer unit) is used to cut the half-folded sheet bunch 301, first, skew feed correction of the half-folded sheet bunch 301, which is folded about the half-folded portion in the shape of a brochure, is made by an appropriate registration member. After that, the sheet bunch 301 is conveyed to a predetermined position and the relative position thereof with respect to a cutting blade is adjusted. After that, the cutting blade shears the half-folded sheet bunch 301 having an appropriate set cutting margin so that the edge portions (fore edge) 301a of the spread of the half-folded sheet bunch 301 are cut.
However, the entire size of a conventional sheet cutting apparatus (trimmer unit) is very large in order that the sheet cutting apparatus have various functions, and hence, in some cases, such a sheet cutting apparatus cannot be placed in limited spaces, such as an office. Thus, under the present circumstances, such a sheet cutting apparatus is not popularly used. A main reason of such an increase in size of a sheet cutting apparatus is that, in particular, a distance for storing the half-folded sheet bunch is necessary between the cutting blade as the cutting member and the above-mentioned registration member. More specifically, the distance between the cutting blade and the registration member for storing the sheet bunch needs to be set longer than at least the maximum size of the half-folded sheet bunch, which makes large the length of the apparatus as a whole in a conveying direction of the half-folded sheet bunch and, eventually, makes large the apparatus as a whole.
Immediately before such a half-folded sheet bunch is cut, in the sheet cutting apparatus, for example, an appropriate plate-like pressing member presses flatly the whole half-folded sheet bunch so that air is removed from the inside of the half-folded sheet bunch. However, in a conventional sheet cutting apparatus, in order to press flatly the whole half-folded sheet bunch, the pressing member is formed of a large plate-like member, which is another reason that the apparatus itself is increased in size.
For example, in an image forming system in which a sheet post-processing apparatus B2 is attached to an image forming apparatus A2 as illustrated in FIG. 13, a trimmer unit C2 is disposed so as to protrude from a lower end portion of the sheet post-processing apparatus B2 toward a downstream side. However, if such a large trimmer unit C2 is attached, not only the footprint of the image forming system as a whole becomes extremely large but also there are other problems including reduced amount of sheets which can be stacked on a sheet stacking tray 303 provided to the sheet post-processing apparatus B2 because the movable range in a vertical direction of the sheet stacking tray 303 becomes small.